Magnus was the son of Earl Erlend of Orkney, of the line of Rögnvald, and a kinsman of the kings of Norway. In a sea-raid in his youth, ordered against Anglesey by King Magnus Barefoot of Norway, he famously refused to fight — sitting on the prow and chanting psalms while the battle raged — for which he was branded a coward and forced ashore. He returned home a Christian penitent.
On the death of his father he shared the earldom of Orkney with his cousin Haakon Paulsson. The shared rule grew strained, and in 1117 the two cousins met for a peace conference on the island of Egilsay, each having sworn to bring only two ships. Haakon arrived with eight. Knowing what was intended, Magnus spent the night in the small Egilsay church in prayer, then offered himself for execution in place of Haakon's followers. His cousin's cook was forced to kill him with a single axe-blow to the head.
Miracles were immediately reported at his grave at Birsay; Bishop William of Orkney, initially a skeptic, confirmed the cult after his own healing. Magnus's nephew Rögnvald began the great Romanesque cathedral at Kirkwall in his honour in 1137 — St. Magnus Cathedral remains the northernmost medieval cathedral in the British Isles and his relics rest in one of its piers.